ABSTRACT Trait‐based ecology seeks to improve plant growth predictions by incorporating individual trait variability, yet empirical studies have shown that even individual‐level traits struggle to predict growth due to unclear mechanisms. In a 3‐year experiment measuring 10 functional traits and growth in 5188 seedlings, we found that individual trait–growth relationships are strongly competition‐dependent. Without competition, traits strongly predicted growth, with predictive ability positively associated with trait variability, supporting theoretical expectations. However, competition significantly reduced the predictive ability of traits by simultaneously increasing individual trait variability and suppressing seedling growth rates. These results suggest that plants under competition achieve similar growth through diverse alternative strategies, obscuring trait–growth relationships. Our findings suggest that competition‐induced trait variability might be a key driver of the weak trait–growth relationships frequently observed in natural communities, providing experimental evidence for the ongoing debate surrounding the weak trait–growth correlations.